# Fin Rot in Fish: Causes, Symptoms and How to Treat It

> Fin rot is a bacterial infection that erodes the fins, often starting at the edges with a frayed, white or red border. It is usually triggered by poor water quality or stress. Improve water conditions first, move the fish to a hospital tank, and use an antibiotic if the damage keeps spreading.

Source: https://petremedyusa.com/guides/fin-rot-treatment-guide/ · Updated 2026-09-30 · PetRemedyUSA

## What fin rot looks like

Healthy fins are clear, smooth and fully extended. Fin rot begins at the tips or edges and works toward the body. Look for:

- Fins that look ragged, frayed or shortened
- White, milky or black edges on the fins
- Red streaks or inflamed skin at the fin base
- Clamped fins and reduced appetite in advanced cases

If the rot reaches the base of the fin and the body, treatment becomes far harder, so early action matters.

## Why fish get fin rot

The bacteria involved, commonly *Aeromonas*, *Pseudomonas* and related species, are present in most aquariums. They cause disease when a fish is weakened. Typical triggers are:

- High ammonia, nitrite or nitrate from overdue water changes or an overloaded filter
- Fin nipping or aggression from tank mates
- Sudden temperature swings
- Sharp decorations that tear fins
- Overcrowding and chronic stress

## Step 1: Fix the water first

Test ammonia, nitrite and nitrate. Ammonia and nitrite should read zero. Perform a partial water change with dechlorinated, temperature-matched water and repeat as needed. Mild fin rot often stops progressing once water quality is corrected, because the fish's own defenses recover.

## Step 2: Use a hospital tank if it is spreading

If the damage continues after a few days of clean water, move the fish to a [hospital tank](https://petremedyusa.com/guides/hospital-tank-setup/). Treating there protects the beneficial bacteria in your main filter and lets you measure medication accurately for a smaller volume of water.

## Step 3: Choose an antibiotic class

Fin rot is bacterial, so an antibiotic is the appropriate medication when water changes alone are not enough. Aquarists commonly use:

| Option | Best known for |
| --- | --- |
| [Amoxicillin](https://petremedyusa.com/medications/amoxicillin-for-fish/) | Mild to moderate infections with a gram-positive component |
| [Cephalexin](https://petremedyusa.com/medications/cephalexin-for-fish/) | Skin, fin and wound infections |
| [Ciprofloxacin](https://petremedyusa.com/medications/ciprofloxacin-for-fish/) | Aggressive or fast-moving gram-negative infections |

Our [antibiotic comparison guide](https://petremedyusa.com/guides/fish-antibiotics-compared/) explains how the classes differ. Always follow the directions on the label, complete the full course, and do not stop early because the fish looks better.

## Supporting recovery

- Keep the hospital tank warm and stable, and provide hiding places.
- Remove activated carbon while medicating, because it absorbs medication.
- Feed small, high-quality meals and remove uneaten food promptly.
- Watch for regrowth. Fins can regenerate once the infection is controlled and the water is clean.

## Preventing a repeat

Keep up regular water changes, avoid overstocking, choose compatible tank mates and quarantine new fish. Prevention is far cheaper than treatment.

> **When to call a professional.** If several fish are affected, the fish stops eating, or you are unsure of the diagnosis, consult an aquatic veterinarian.

## Frequently asked questions

### Can fin rot heal on its own?

Mild fin rot can improve once water quality is corrected and stress is removed. If it keeps spreading or reaches the base of the fin, a bacterial infection has taken hold and antibiotic treatment is usually needed.

### How long does fin rot take to heal?

The infection may stop progressing within several days of proper care, but full fin regrowth often takes weeks. Speed depends on the fish, water quality and how early treatment started.

### Is fin rot contagious to other fish?

The bacteria are already in most tanks, but a heavily infected fish can shed more of them, and stressed tank mates may also become ill. Isolating the affected fish in a hospital tank is the safest approach.

### Which antibiotic is best for fin rot?

There is no single best choice. Amoxicillin and cephalexin are common for milder cases, while ciprofloxacin is often chosen when the infection is aggressive or gram-negative. An aquatic veterinarian can advise for your species.